Semiconductor transistors with expanded top portions of gates

ABSTRACT

A semiconductor transistor with an expanded top portion of a gate and a method for forming the same. The semiconductor transistor with an expanded top portion of a gate includes (a) a semiconductor region which includes a channel region and first and second source/drain regions; the channel region is disposed between the first and second source/drain regions, (b) a gate dielectric region in direct physical contact with the channel region, and (c) a gate electrode region which includes a top portion and a bottom portion. The bottom portion is in direct physical contact with the gate dielectric region. A first width of the top portion is greater than a second width of the bottom portion. The gate electrode region is electrically insulated from the channel region by the gate dielectric region.

This application is a divisional application claiming priority to Ser.No. 11/275,514, filed Jan. 11, 2006.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to semiconductor transistors, and moreparticularly, to semiconductor transistors with expanded top portions ofgates.

2. Related Art

In the fabrication process of a typical semiconductor device, if a gateis small it is very difficult to form silicide in the top portion of thegate. Therefore, there is a need for a semiconductor transistor with anexpanded top portion of a gate (and a method for forming the same).

SUMMARY OF THE INVENTION

The present invention provides a semiconductor structure, comprising (a)a semiconductor region including a channel region, a first source/drainregion, and a second source/drain region, wherein the channel region isdisposed between the first source/drain region and the secondsource/drain region; (b) a gate dielectric region in direct physicalcontact with the channel region; and (c) a gate electrode regionincluding a top portion and a bottom portion, wherein the bottom portionis in direct physical contact with the gate dielectric region, wherein afirst width of the top portion is greater than a second width of thebottom portion, wherein the gate electrode region is electricallyinsulated from the channel region by the gate dielectric region, andwherein a first upper portion and a second upper portion of the firstand second source/drain regions, respectively, are compressivelystrained.

The present invention provides a semiconductor structure, comprising (a)a semiconductor region including a channel region, a first source/drainregion, and a second source/drain region, wherein the channel region isdisposed between the first source/drain region and the secondsource/drain region; (b) a gate dielectric region in direct physicalcontact with the channel region; (c) a gate electrode region including atop portion and a bottom portion, wherein the bottom portion is indirect physical contact with the gate dielectric region, wherein a firstwidth of the top portion is greater than a second width of the bottomportion, and wherein the gate electrode region is electrically insulatedfrom the channel region by the gate dielectric region; and (d) an ionbeam incident on the gate electrode region, wherein the ion beamcomprises ions of a material selected from the group consisting ofgermanium and arsenic.

The present invention provides a semiconductor transistor with anexpanded top portion of a gate or an expanded top portion of a source ordrain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-10 show a first fabrication process of a semiconductortransistor with an expanded top portion of a gate, in accordance withembodiments of the present invention.

FIGS. 11-20 show a second fabrication process of a verticalsemiconductor transistor with an expanded top portion of a gate, inaccordance with embodiments of the present invention.

FIGS. 21-30 show a third fabrication of another semiconductor transistorwith an expanded top portion of a gate, in accordance with embodimentsof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-10 show a first fabrication process for forming a transistorstructure 100, in accordance with embodiments of the present invention,wherein FIGS. 1-10 show cross-section views of the transistor structure100.

More specifically, with reference to FIG. 1, in one embodiment, thefirst fabrication process starts out with a silicon substrate 110.

Next, with reference to FIG. 2, in one embodiment, two trenches 210 and220 are formed in the silicon substrate 110. Illustratively, thetrenches 210 and 220 are formed using a conventional lithographic andetching process.

Next, with reference to FIG. 3, in one embodiment, two STI (ShallowTrench Isolation) regions 310 and 320 are formed in the two trenches 210and 220 (FIG. 2), respectively, using a conventional method.Illustratively, the two STI regions 310 and 320 comprise silicondioxide.

Next, with reference to FIG. 4, in one embodiment, a gate dielectriclayer 410 is formed on a top surface 111 of the silicon substrate 110.Illustratively, the gate dielectric layer 410 comprises silicon dioxide.In one embodiment, the gate dielectric layer 410 is formed by thermaloxidation.

Next, with reference to FIG. 5, in one embodiment, a gate electroderegion 510 is formed on the top surface 111 of the silicon substrate110. In one embodiment, the gate electrode region 510 is formed by (i)CVD (Chemical Vapor Deposition) of polysilicon everywhere on a topsurface 412 of the structure 100 (FIG. 4) to form a polysilicon layer(not shown), and then (ii) a conventional lithographic and etchingprocess to etch the deposited polysilicon layer, resulting in the gateelectrode region 510, as shown in FIG. 5.

Next, with reference to FIG. 6, in one embodiment, extension regions 610and 620 are formed in the silicon substrate 110. Illustratively, theextension regions 610 and 620 are formed by ion implantation using thegate electrode region 510 as a blocking mask.

Next, with reference to FIG. 7, in one embodiment, halo regions 710 and720 are formed in the silicon substrate 110. Illustratively, the haloregions 710 and 720 are formed by ion implantation using the gateelectrode region 510 as a blocking mask.

Next, with reference to FIG. 8, in one embodiment, dielectric spacers810 and 820 are formed on side walls of the gate electrode region 510.Illustratively, the dielectric spacers 810 and 820 are formed by (i) CVDof an insulating material, such as silicon dioxide or silicon nitride,or a composite, everywhere on top of the structure 100 of FIG. 7, andthen (ii) directional etching back until the top surface 111 of thesilicon substrate 110 and a top surface 511 of the gate electrode region510 are exposed to the surrounding ambient.

Next, in one embodiment, source/drain regions 840 and 850 are formed inthe silicon substrate 110. Illustratively, the source/drain regions 840and 850 are formed by ion implantation using the gate electrode region510 and the dielectric spacers 810 and 820 as a blocking mask.

Next, in one embodiment, germanium atoms are implanted in a top portion512 of the gate electrode region 510 by ion implantation in a directionindicated by arrows 830. Hereafter, the implantation of germanium atomsin the top portion 512 of the gate electrode region 510 of FIG. 8 can bereferred to as a germanium implantation step 830. Illustratively, thegermanium implantation step 830 uses germanium atoms at a high dose(10¹⁶ Ge atoms/cm²) and at a low energy. The directions 830 can bevertical or tilted less than 10 degrees from vertical. As a result ofthe germanium implantation step 830, the top portion 512 expandslaterally, as shown in FIG. 9A.

With reference to FIG. 9A, it can be seen that as a result of thelateral expansion of the top portion 512, a width 517 of the top portion512 is greater than a width 516 of the bottom portion 515. In oneembodiment, the top portion 512 of the gate electrode region 510 isexpanded laterally at least 20%. In other words, the width 517 is atleast 120% of the width 516.

Next, with reference to FIG. 9B, in one embodiment, a metal (e.g.,nickel, etc.) layer 910 is formed on top of the structure 100 of FIG.9A. Illustratively, the nickel layer 910 is formed by sputtering ofnickel everywhere on top of the structure 100 of FIG. 9A.

Next, with reference to FIG. 10, in one embodiment, silicide regions512, 1010, and 1020 are formed on top of the gate electrode region 510,the source/drain regions 840 and 850, respectively. Illustratively, thesilicide regions 512, 1010 and 1020 comprise nickel silicide. In oneembodiment, the silicide regions 512, 1010 and 1020 are formed by firstannealing the whole structure 100 of FIG. 9B so that nickel of thenickel layer 910 chemically reacts with silicon of the gate electroderegion 510, the source/drain regions 840 and 850, resulting in thesilicide regions 512, 1010 and 1020. Then, in one embodiment, unreactednickel is removed by a wet etching step, resulting in structure 100 ofFIG. 10. In one embodiment, the entire top portion 512 (FIG. 9B) of thegate electrode region 510 chemically reacts with Ni of the Ni layer 910resulting in the silicide region 512 as sown in FIG. 10.

As can be seen in FIGS. 8, 9B, and 10, because of the germaniumimplantation step 830 (FIG. 8), an interfacing surface 514 between thenickel layer 910 and the top portion 512 of the gate electrode region510 (FIG. 9B) is larger than the case in which the implantation step 830is not performed. Therefore, it is easier for nickel (of the nickellayer 910) to react with silicon of the top portion 512 (FIG. 9B) thanin the case the top portion of the gate electrode region 510 is notexpanded. Also as a result of the top portion 512 being expandedlaterally, the silicide region 512 (FIG. 10) is more conductive than thecase in which the top portion 512 of the gate electrode 510 is notexpanded.

FIGS. 11-20 show a second fabrication process for forming a transistorstructure 200, in accordance with embodiments of the present invention.

More specifically, with reference to FIG. 11, in one embodiment, thesecond fabrication process starts out with an SOI (Silicon on Insulator)substrate 1110. Illustratively, the SOI substrate 1110 comprises asilicon layer 1120, a buried oxide layer 1130 on the silicon layer 1120,and a silicon layer 1140 on the buried oxide layer 1130. Illustratively,the SOI substrate 1110 is formed by a conventional method. In oneembodiment, the SOI substrate 1110 may comprise an Ultra-Thin SOIwherein the silicon layer 1140 is less than 15 nm in thickness.

Next, in one embodiment, a dielectric hard mask layer 1150 is formed ontop of the silicon layer 1140. Illustratively, the dielectric hard masklayer 1150 is formed by CVD of silicon nitride or silicon dioxide, or acomposite of the two, everywhere on top of the silicon layer 1140.

Next, in one embodiment, a lithographic and etching step is performed toetch the dielectric hard mask layer 1150 and then the silicon layer 1140so as to form a dielectric cap region 1151 and a fin region 1141,respectively, as shown in FIG. 12.

With reference to FIG. 12 (a front view of the structure 200), it shouldbe noted that the dielectric cap region 1151 and the fin region 1141 arefarther away from the viewer than the silicon layer 1120 and the buriedoxide layer 1130.

Next, with reference to FIG. 13A, in one embodiment, a silicon dioxidelayer 1310 is formed on side walls of the fin region 1141 of FIG. 12.Illustratively, the silicon dioxide layer 1310 is formed by thermaloxidation. FIG. 13A shows a front view of the structure 200 after thesilicon dioxide layer 1310 is formed. In alternative embodiments, 1310may comprise a high-k gate dielectric, such as hafnium silicate,deposited, for example, by means of CVD, MOCVD, ALD.

Next, with reference to FIG. 13B, in one embodiment, a gate electroderegion 1320 is formed on top of the dielectric cap region 1151 and onside walls of the silicon dioxide layer 1310. Illustratively, the gateelectrode region 1320 comprises polysilicon. In one embodiment, the gateelectrode region 1320 is formed by (i) CVD of polysilicon everywhere ontop of the structure 200 of FIG. 13A, and then (ii) a conventionallithographic and etching process. FIG. 13B shows a front view of thestructure 200 after the gate electrode region 1320 is formed. So, itshould be noted that the silicon dioxide layer 1310 and the dielectriccap region 1151 are farther away from the viewer than the gate electroderegion 1320.

Next, in one embodiment, extension regions 1410 and 1420 and haloregions 1430 and 1440 (not shown in FIG. 13B but can be seen in FIG. 14)are formed in the fin region 1141 of FIG. 12 by ion implantation usingthe gate electrode region 1320 as a blocking mask.

FIG. 14 shows a top down view of the structure 200 of FIG. 13B along aline 14-14 after the formation of the extension regions 1410 and 1420and halo regions 1430 and 1440.

Next, in one embodiment, germanium atoms are implanted on a top portion1321 (FIG. 13B) of the gate electrode region 1320 by ion implantation.Illustratively, germanium atoms are implanted at a high dose (10¹⁶ Geatoms/cm²) and at a low energy. As a result of the germaniumimplantation in the top portion 1321 (FIG. 13B) of the gate electrode1320, the top portion 1321 expands laterally as shown in FIG. 15.

With reference to FIG. 15, it can be seen that as a result of thelateral expansion of the top portion 1321, a width 1326 of the topportion 1321 is greater than a width 1325 of a bottom portion 1322. Inone embodiment, the top portion 1321 of the gate electrode region 1320is expanded laterally at least 20%. In other words, the width 1326 is atleast 120% of the width 1325.

Next, with reference to FIG. 16, in one embodiment, a silicon dioxidelayer 1610 is formed on top and side walls of the gate electrode region1320. Illustratively, the silicon dioxide layer 1610 is formed bythermal oxidation. Hereafter, expanded top portions 1620 and 1630 of thegate electrode region 1320 are referred to as overhangs 1620 and 1630.FIG. 16 shows a front view of the structure 200 after the silicondioxide layer 1610 is formed (except for the silicon dioxide layer 1610and the gate electrode region 1320 whose cross section view is shown).It should be noted that, the silicon dioxide layer 1310 and thedielectric cap region 1151 are farther away from the viewer than thesilicon dioxide layer 1610 and the gate electrode region 1320.

Next, with reference to FIG. 17, in one embodiment, dielectric spacers1710 and 1720 are formed on side walls of the gate electrode region 1320and under the overhangs 1620 and 1630. Illustratively, the dielectricspacers 1710 and 1720 are formed by (i) CVD of a dielectric material,such as silicon dioxide, silicon nitride, or a composite of the two,everywhere on top of the structure 200 of FIG. 16 to form a dielectriclayer (not shown), and then (ii) directionally etching back thedeposited dielectric layer. More specifically, the deposited dielectriclayer is over etched so that the dielectric spacers 1710 and 1720 remainon side walls of the gate electrode region 1320 but no dielectricmaterial remains on side walls of the silicon dioxide layer 1310. FIG.17 shows a front view of the structure 200 after the dielectric spacers1710 and 1720 are formed (except for the silicon dioxide layer 1610, thegate electrode region 1320 and the dielectric spacers 1710 and 1720whose cross section view is shown).

Next, in one embodiment, source/drain regions 1810 and 1820 (not shownin FIG. 17 but can be seen in FIG. 18) are formed in the fin region 1141of FIG. 18 by ion implantation using the gate electrode region 1320 andthe dielectric spacers 1710 and 1720 as a blocking mask.

FIG. 18 shows a top down view of the structure 200 of FIG. 17 along aline 18-18 after the formation of the source/drain regions 1810 and1820.

Next, with reference to FIG. 19, in one embodiment, the dielectric capregion 1151 of FIG. 17 is removed by a Reactive Ion Etch (RIE), or a wetetching step, resulting in the structure 200 of FIG. 19.

Next, with reference to FIG. 20, in one embodiment, silicide regions2010, 2020, and 2030 are formed on top of the gate electrode region 1320and the source/drain regions 1810 and 1820 (FIG. 18). Illustratively,the silicide regions 2010, 2020, and 2030 comprise silicide nickel. Inone embodiment, the silicide regions 2010, 2020 and 2030 are formed by(i) sputtering of nickel everywhere on top of the structure 200 (FIG.19) to form a nickel layer (not shown), then (ii) annealing so thatnickel of the deposited nickel layer chemically reacts with silicon ofthe gate electrode region 1320 and the source/drain regions 1810 and1820 (FIG. 18) resulting in the silicide regions 2010, 2020, and 2030.Then, unreacted nickel is removed by a wet etching step, resulting instructure 200 of FIG. 20.

Similar to the structure 100 of FIG. 10, the structure 200 of FIG. 20has an advantage of the enlarged silicide region 2010 which is moreconductive than in the case in which the top portion 1321 of the gateelectrode 1320 is not expanded laterally by the germanium implantation.Moreover, because the top portion 1321 of the gate electrode 1320 (FIG.19) is enlarged, it is easier for nickel of the deposited nickel layer(not shown) to chemically react with silicon of the gate electroderegion 1320 to form the silicide 2010.

FIGS. 21-30 show a third fabrication process for forming a transistorstructure 300, in accordance with embodiments of the present invention,wherein FIGS. 21-30 show cross-section views of the transistor structure300.

More specifically, with reference to FIG. 21, in one embodiment, thethird fabrication process starts out with an SOI substrate 2110. In oneembodiment, the SOI substrate 2110 comprises a silicon layer 2120, aburied oxide layer 2130 on the silicon layer 2120, and a silicon layer2140 on the buried oxide layer 2130. Illustratively, the SOI substrate2110 is formed by a conventional method.

Next, with reference to FIG. 22, in one embodiment, a trench 2210 isformed in the silicon layer 2140. In one embodiment, the trench 2210 isformed by a conventional lithographic and etching process.

Next, with reference to FIG. 23, in one embodiment, an STI region 2310is formed in the trench 2210 (FIG. 22) using a conventional method.Illustratively, the STI region 2310 comprises silicon dioxide.

Next, with reference to FIG. 24, in one embodiment, a gate dielectriclayer 2410 is formed on top of the structure 300 (FIG. 23). The gatedielectric layer 2410 may be formed (a) by oxidation and nitridation ofa top portion of the silicon layer 2140, to form a silicon oxinitridedielectric, or (b) by deposition of a high-k material such as hafniumsilicate by CVD, MOCVD, or ALD.

Next, with reference to FIG. 25, in one embodiment, a polysilicon layer2510 is formed on top of the structure 300 (FIG. 24) by CVD.

Next, in one embodiment, the polysilicon layer 2510 is selectivelyetched, resulting in a gate electrode region 2511 as shown in FIG. 26.

Next, with reference to FIG. 26, in one embodiment, extension regions2610 and 2620 and halo regions 2630 and 2640 are formed in the siliconlayer 2140. Illustratively, the extension regions 2610 and 2620 and haloregions 2630 and 2640 are formed by ion implantation using the gateelectrode region 2511 as a blocking mask. Hereafter, a silicon region ofthe silicon layer 2140 which is disposed between the extension regions2610 and 2620 and the halo regions 2630 and 2640 is referred to as achannel region 2140.

Next, with reference to FIG. 27, in one embodiment, dielectric spacers2710 and 2720 are formed on side walls of the gate electrode region2511. Illustratively, the dielectric spacers 2710 and 2720 are formed by(i) CVD of a dielectric layer, such as silicon dioxide or siliconnitride, or a composite of both, everywhere on top of the structure 300of FIG. 26, and then (ii) directional etching back. Any remaining gatedielectric layer 2410 in the etched-back regions is completely removedby either sufficient over etch, or by and additional etching process,resulting in a gate dielectric region 2411.

Next, with reference to FIG. 28A, in one embodiment, silicon regions2810 and 2820 are epitaxially grown on the extension regions 2610 and2620, respectively.

It should be noted that the silicon is also epitaxially grown on top ofthe gate electrode region 2511. But to make the description simple, thisis not shown. Alternatively, in one embodiment, before the formation ofthe silicon regions 2810 and 2820 by epitaxial growth, a cap region (notshown) can be formed on top of the gate electrode region 2511. In oneembodiment, the cap region (not shown) comprises a silicon dioxide layerand a silicon nitride layer (not shown). More specifically, the silicondioxide layer and the silicon nitride layer (not shown) can be formed inthat order on top of the polysilicon layer 2510 of FIG. 25. After that,the silicon dioxide layer and the silicon nitride layer (not shown) canbe patterned at the same time that the gate electrode region 2511 isformed. As a result, portions of the silicon dioxide layer and thesilicon nitride layer (not shown) still remain on top of the gateelectrode region 2511. Therefore, the cap region (not shown) can preventepitaxial growth of the silicon on top of the gate electrode region2511.

Next, in one embodiment, the gate electrode region 2511 and thedielectric spacers 2710 and 2720 are used as a blocking mask to ionimplant the silicon regions 2810 and 2820, the extension regions 2610and 2620 and the halo regions 2630 and 2640 so as to form source/drainregions 2811 and 2821 (as shown in FIG. 28B).

Next, in one embodiment, with reference to FIG. 28B, germanium atoms areimplanted in a top portion 2512 of the gate electrode region 2511 by ionimplantation in a direction indicated by arrows 2830. Hereafter, theimplantation of germanium atoms in the top portion 2512 of the gateelectrode region 2511 can be referred to as a germanium implantationstep 2830. Illustratively, the germanium implantation step 2830 usesgermanium atoms at a high dose (10¹⁶ Ge atoms/cm²) and at a low energy.As a result of the germanium implantation step 2830, the top portion2512 expands laterally, as shown in FIG. 29.

With reference to FIG. 29, it can be seen that as a result of thelateral expansion of the top portion 2512, a width 2519 of the topportion 2512 is greater than a width 2518 of a bottom portion 2514. Inone embodiment, the top portion 2512 of the gate electrode region 2511is expanded laterally at least 20%. In other words, the width 2519 is atleast 120% of the width 2518. In one embodiment, the germaniumimplantation step 2830 also implants Germanium atoms in upper portions2811 a and 2821 a of the source/drain regions 2811 and 2821,respectively. As a result, the upper portions 2811 a and 2821 a areexpanded laterally and compressively strained. Therefore, the channelregion 2140 is tensile strained.

Next, with reference to FIG. 30, in one embodiment, silicide regions2513, 2812 and 2822 are formed on top of the gate electrode region 2511,the source/drain regions 2811 and 2821, respectively. Illustratively,the silicide regions 2513, 2812, and 2822 comprise silicide nickel. Inone embodiment, the silicide regions 2513, 2811 and 2821 are formed by(i) CVD of nickel everywhere on top of the structure 300 (FIG. 29) toform a nickel layer (not shown), then (ii) annealing so that thedeposited nickel layer chemically reacts with silicon on top portions ofthe gate electrode region 2511, the source/drain regions 2811 and 2821so as to form the silicide regions 2513, 2812 and 2822. Then, unreactednickel is removed by a wet etching step, resulting in structure 300 ofFIG. 30.

In the embodiments described above, germanium ions/atoms are implantedin the gates so as to expand the top portions of the gates.Alternatively, arsenic can be used instead of germanium. Also, in oneembodiment, the germanium and arsenic ion implantations can be carriedout at room temperature with the ions being at an energy of 25 KeV suchthat the ions can reach as deep as 23 nm in the gates.

In one embodiment, as a result of the Ge implantation in the top portion512 (FIG. 9A), the top portion 1321 (FIG. 13B), the top portion 2512(FIG. 29), and in the top portions 2811 a and 2821 a (FIG. 29), each ofthese portions 512, 1321, 2512, 2811 a, and 2821 a is at least 0.5%compressively strained, meaning the average atom spacing of theresulting Si—Ge lattice is 0.5% less than the average atom spacing of aSi—Ge mixture of the same composition ratio in a relaxed/unstrainedcondition.

While particular embodiments of the present invention have beendescribed herein for purposes of illustration, many modifications andchanges will become apparent to those skilled in the art. Accordingly,the appended claims are intended to encompass all such modifications andchanges as fall within the true spirit and scope of this invention.

The invention claimed is:
 1. A semiconductor structure, comprising: (a)a semiconductor region including a channel region, a first source/drainregion, and a second source/drain region, wherein the channel region isdisposed between the first source/drain region and the secondsource/drain region, and wherein a first direction from thesemiconductor region to the channel region is perpendicular to a topsurface of the semiconductor region; (b) a gate dielectric region indirect physical contact with the channel region; and (c) a gateelectrode region including a top portion and a bottom portion, whereinthe bottom portion is in direct physical contact with the gatedielectric region, wherein the gate electrode region is electricallyinsulated from the channel region by the gate dielectric region, andwherein a first upper portion and a second upper portion of the firstand second source/drain regions are wider in a second directionperpendicular to the first direction than are a first lower portion anda second lower portion of the first and second source/drain regions,respectively; wherein the first upper portion of the first source/drainregion consists of a first upper layer and a first lower layer; whereinthe first upper layer comprises a first silicide and an implantedmaterial; wherein the first lower layer comprises silicon and theimplanted material and does not comprise the first silicide; wherein thesecond upper portion of the second source/drain region consists of asecond upper layer and a second lower layer; wherein the second upperlayer comprises a second silicide and the implanted material; whereinthe second lower layer comprises silicon and the implanted material anddoes not comprise the second silicide, wherein the first upper layer isabove the gate dielectric region in the first direction such that thegate dielectric region is disposed in the first direction between thechannel region and the first upper layer, and wherein the second upperlayer is above the gate dielectric region in the first direction suchthat the gate dielectric region is disposed in the first directionbetween the channel region and the second upper layer.
 2. The structureof claim 1, wherein the first lower layer does not comprise the firstsilicide, and wherein the second lower layer does not comprise thesecond silicide.
 3. The structure of claim 2, wherein the implantedmaterial comprises germanium or arsenic.
 4. The structure of claim 1,wherein the implanted material comprises germanium or arsenic.
 5. Thestructure of claim 1, wherein the wherein each of the first upperportion and the second upper portion of the first and secondsource/drain regions, respectively, is compressively strained.
 6. Thestructure of claim 1, said structure further comprising: a buried oxidelayer in direct physical contact with both the top surface of thesemiconductor region and a bottom surface of the channel region, whereinthe buried oxide layer is disposed in the first direction between thesemiconductor region and the channel region, and wherein the channelregion is disposed in the first direction between the buried oxide layerand the gate dielectric region.
 7. The structure of claim 1, wherein thefirst upper layer has a width in the second direction that ismonotonically increasing in the first direction from a bottom surface ofthe first upper layer to a top surface of the first upper layer; whereinthe first lower layer has a width in the second direction that ismonotonically increasing in the first direction from a bottom surface ofthe first lower layer to a top surface of the first lower layer; whereinthe bottom surface of the first upper layer and the top surface of thefirst lower layer coincide with each other and have a same width in thesecond direction; wherein the first lower portion of the firstsource/drain region has a width in the second direction that is constantwith respect to the first direction; wherein the second upper layer hasa width in the second direction that is monotonically increasing in thefirst direction from a bottom surface of the second upper layer to a topsurface of the second upper layer; wherein the second lower layer has awidth in the second direction that is monotonically increasing in thefirst direction from a bottom surface of the second lower layer to a topsurface of the second lower layer; wherein the bottom surface of thesecond upper layer and the top surface of the second lower layercoincide with each other and have a same width in the second direction;wherein the second lower portion of the second source/drain region has awidth in the second direction that is constant with respect to the firstdirection.